Maximum directivity from a horn antenna may be obtained by uniform amplitude and phase distribution over the horn aperture. Such horns are denoted as “hard” horns.
Exemplary hard horns may include one having longitudinal conducting strips on a dielectric wall lining, and the other having longitudinal corrugations filled with dielectric material. These horns work for various aperture sizes, and have increasing aperture efficiency for increasing size as the power in the wall area relative to the total power decreases.
Dual mode and multimode horns like the Box horn can also provide high aperture efficiency, but they have a relatively narrow bandwidth, in particular for circular polarization. Higher than 100% aperture efficiency relative to the physical aperture may be achieved for endfire horns. However, these endfire horns also have a small intrinsic bandwidth and may be less mechanically robust.
Linearly polarized horn antennas may exist with high aperture efficiency at the design frequency, large bandwidth and low cross-polarization. However, these as well as the other non hybrid-mode horns only work for limited aperture size, typically under 1.5 or 2λ.